1. Field of the Invention
The present invention generally relates to a method for optically recording information and more particularly a method for optically recording information in which a multi level signal is recorded on a track.
2. Description of the Related Art
The Japanese Laid-Open Patent Application No.8-147695 (a prior art method {circle around (1)}) discloses such kind of technology. In the prior art method {circle around (1)}, although the power of a laser diode is not decided in an ALPC zone, the laser diode is driven by a simple low bandwidth drive circuit with a multiple level drive current. First, the laser diode is driven on a trial basis in a Test Zone or a Manufacturer Zone, which zones are placed at an inside and an outside of an optical disc, and each zone has more than 100 tracks. In these zones, first, the laser diode is driven by the drive circuit having a current level of Iw0. Next, the laser diode is driven by the drive circuit having a current level of Iw1. Then, the laser diode is driven by the drive circuit having a current level of Iw2. Then, each current level of Iw0, Iw1 and Iw2 is optimally determined for predetermined power levels, respectively. When information is written on the disc in a write mode, first, a w0 D/A converter is supplied with data for the current value Iw0, a w1 D/A converter is supplied with data for the current value Iw1 and a w2 D/A converter is supplied with data for the current value Iw2. Then, switches SW4, SW5 and SW6 are turned on or turned off according to the data to be written. As a result, the laser diode LD1 is supplied with the LD drive current.
The Japanese Laid-Open Patent Application No. 11-25456 (a prior art method {circle around (2)}) also discloses such kind of technology. The prior art method {circle around (2)} discloses a simple method for recording multi level information on an optical information medium in order to raise recording density. In the prior art method {circle around (2)}, initially, first pits are formed on a re-writable media (a phase change optical disc) by radiating a laser light. Then, second pits are formed on the first pits by erasing parts of the first pits according to a multi leveled signal of the information to be recorded. Sizes of the second pits are modulated according to the multi level information.
FIG. 1 shows an example of a waveform of a recording pulse signal according to the prior art method {circle around (1)} as described above. The waveform of the recording pulse signal is composed of a rectangular recording pulse and an erasing pulse. A means for adjusting the size of a recorded mark controls the width of the recording pulse (T3−T1 as shown in FIG. 1) or strength of a light (Pw as shown in FIG. 1). On reproducing the recorded marks, strength of a reflected light by the mark is varied according to the size of the recorded mark. Therefore, a multi level signal, which is converted from the light reflected and has one of MARK signal levels corresponding to the size of the recorded mark, can be obtained as shown in FIG. 2. FIG. 2 shows a principle of such multi level recording method using a recorded mark size modulation.
A disadvantage of this method using the waveform of the recording pulse signal composed of the rectangular recording pulse and the erasing pulse is that the recorded mark is apt to be enlarged in the radial direction because of thermal diffusion caused by the laser beam focused on the optical information medium. Therefore, the shape of the recorded mark is apt to have a shape like a drop of water as shown in FIG. 1. Therefore, this method can not control the influence of the thermal diffusion on the size of the recorded mark if the size of the mark smaller than the size of the focused laser beam spot is formed.
FIG. 3 shows a relation between an occupied mark ratio and a multi level mark signal level according to the prior art method {circle around (1)}. As shown in FIG. 3, at a region {circle around (1)} in which the mark signal level is relatively high, a ratio of the mark signal level variation to the occupied mark ratio variation is high compared to that at a region {circle around (2)} in which the mark signal level is relatively low. Therefore, there is a need to control the occupied mark ratio more precisely at the region {circle around (1)} than at the region {circle around (2)}. However, the prior art method {circle around (1)} as shown in FIG. 1 cannot control the occupied mark ratio accurately because it cannot control the influence of the thermal diffusion to the size of the recorded mark. Therefore, the prior art method {circle around (1)} cannot control each level of the multi level signal accurately.
FIG. 5 shows the prior art method {circle around (2)}. To eliminate the disadvantage of the prior art method {circle around (1)}, it is possible to use a recording pulse signal composed of multi pulses as shown in FIG. 5. In this prior art method {circle around (2)}, the recording pulse is composed of a start pulse, short multi pulses and an erasing pulse. As the short multi pulses are used, the enlargement of the recorded mark in the radial direction caused by thermal diffusion can be prevented. A means for adjusting the size of the recorded mark controls the width of the first pulse (T2−T1 as shown in FIG. 5), the duty of the multi pulses (T4−T3 as shown in FIG. 5), or strength of the first pulse and the multi pulses (Pw as shown in FIG. 5).
There is a need to raise the frequency of the recording pulses to form the recorded marks having a size less than that of the focused laser beam spot. Therefore, a recording pulse generation circuit and a laser diode need to be operated at a high frequency in order to record the information on the optical information medium at a high speed. However, it is hard to raise their operating frequencies. Further, the recording pulse generation circuit becomes complex.